A radiation therapy system is provided having a radiation source for providing radiation for treating a patient, including treating skin cancer, wherein the radiation source features a filtering device that changes its filtering characteristic for filtering the radiation for treating the patient in response to a control signal. The filtering device includes two filters and a radiation blocker circumferentially arranged on a cylindrical member. The filtering characteristic is changed by rotating the filtering device in response to the control signal. The filtering device includes a driving unit for rotating the filtering device in response to the control signal in order to change its filtering characteristic, such as a motor with a gearing arrangement for coupling to a corresponding gearing arrangement on the filtering device. In this embodiment, the means for providing the radiation, i.e. the x-ray tube, is arranged inside the filtering device.